Grating writing system

ABSTRACT

A method of forming a complete grating structure on a photosensitive waveguide is disclosed comprising the steps of: (a) writing an initial portion of the grating structure on the waveguide; (b) testing the properties of the initial portion to determine a series of parameters of the initial portion; (c) utilizing the parameters to alter the characteristics of a subsequently written portion of the grating structure to provide for an improved form of grating structure; (d) iterating the steps (a) to (c) so as to form the complete grating structure. The writing can be performed utilizing a coherence pattern formed from the interference of two coherent beams on the waveguide. The characteristics can include the intensity or phase of the subsequently written portion. The testing may include determining the spectral reflectance response of the initial portion or determining the spectral phase delay of the initial portion.

FIELD OF THE INVENTION

The present invention relates to the construction of grating and otherstructures in photosensitive waveguides such as optical fibre devices.

BACKGROUND OF THE INVENTION

The creation of a grating in a photosensitive waveguide materialutilising the interference pattern from two interfering coherent UVbeams is well known. This technique for construction of Bragg gratingsis fully described in U.S. Pat. No. 4,725,110 issued to W H Glenn et.al. and U.S. Pat. No. 4,807,950 issued to W H Glenn et. al.

Bragg grating structures have become increasingly useful and the demandfor longer and longer grating structures having higher and higherquality properties has lead to the general need to create improvedgrating structures.

PCT patent application No. PCT/AU96/00782 by Ouellette et. al. disclosesan improved low noise sensitivity interferometric arrangement whichoperates on a “Sagnac loop” type arrangement.

Australian Provisional Patent Application No. PP3816 by Stepanov et.al., assigned to the present applicant, discloses an advanced form ofinterferometric writing system utilising a modulator to control theposition of an interference pattern on a fibre.

Unfortunately, there is an ever present need for providing furtherimproved grating structures.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide for an improvedgrating writing system.

In accordance with a first aspect of the present invention, there isprovided a method of forming a complete grating structure on aphotosensitive waveguide comprising the steps of: (a) writing an initialportion of the grating structure on the waveguide; (b) testing opticalproperties of the initial portion to determine a series of parameters ofthe initial portion; (c) utilizing the parameters as a feedback to alterthe characteristics of a subsequently written portion of the gratingstructure to provide for an improved form of grating structure; (d)iterating the steps (a) to (c) so as to form the complete gratingstructure.

The writing can be performed utilizing a coherence pattern formed fromthe interference of two coherent beams on the waveguide. Thecharacteristics can include the intensity or phase of the subsequentlywritten portion. The testing may include determining the spectralreflectance transmittance, or group delay response of the initialportion and of subsequently written portions.

The step (a) further can comprise the step of locally perturbing theintensity, phase or frequency of the grating structure and the step (b)can comprise measuring the local perturbation. The local perturbationcan comprise a periodic modulation and the testing step further cancomprise measuring sum and difference frequencies for the modulation.

The testing can comprise averaging portions of the reflectance spectrum,transmittance spectrum and group delay spectrum of the initial portionand of subsequently written portions.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of thepresent invention, preferred forms of the invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings in which:

FIG. 1 illustrates schematically the arrangement of a first embodimentof the present invention; and

FIG. 2 illustrates and arrangement of a second embodiment of the presentinvention.

DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

In the preferred embodiment, the process of writing of a gratingstructure is monitored via a feedback loop and the feedback loop isutilised to alter the writing parameters so as to provide for a highquality grating structure.

In the known systems aforementioned, the grating writing system isutilised to write a local region of the grating and various techniquesdisclosed in the aforementioned applications are utilised to extend thegrating structure. When utilising these methods, a local region of thegrating is typically written to give a local Bragg wavelength L(z) of astrength K(z). The desired wavelength of L(z) can be set by the controlof the interferometric arrangement and the strength K(z) can bedetermined by the residence time of the writing beam on the fibre inaddition to the writing beam strength.

With any physical arrangement however certain errors are likely to occurdue to the non-ideal conditions. For example, variations in theinterferometer may give an error in the pitch. Variations in the laserproperties and the propagating mode effective index are also likely tocause further noise. These variations are generally not predictable andmay have significant effects in the grating properties, for example inthe Q of a high finesse cavity or the group delay ripple of a dispersioncompensator. These errors will lead to an actual Bragg wavelength L'(z)and strength K'(z). Hence the error δL(z) and δK(z) can be given asfollows:

δL(z)=L'(z)−L(z).

δK(z)=K'(z)−K(z).

In the embodiment of the invention a feedback loop is provided so as toprovide for an ongoing monitoring of the state of the grating structure.The feedback loop can be utilised in many different ways but preferablyis provided in a real time manner so as to modify the written spectrumto provide for an improved result.

Turning initially to FIG. 1, there is illustrated a first embodimentwhich comprises an interferometric grating writing apparatus constructedin accordance with the principles as outlined in the aforementioneddisclosure by Glenn et al as modified by Stepanov et al. In thisarrangement 1, a UV laser 2 outputs a beam which is modulated by amodulating element 3. The output from modulator 3 is forwarded to a beamsplitting device 4 which splits the input beam into two beams 5, 6. Thebeams 5, 6 are reflected by mirrors 8, 9 so that they interfere in agrating writing region 10 at which is placed a fibre 11 into which agrating is to be written. In the aforementioned arrangement by Stepanovet al a phase modulator 12 is utilised so as to modify the phase of thetwo beams 5, 6 so that the interference pattern remains stable in thereference frame of the fibre 11 as it is transported in the direction13. In this embodiment, one end 14 of the fibre 11 is interconnected toan analyser 16 which can analyse and digitally record the desiredreflection characteristics. Preferably, the analyser 16 is connected tothe end of fibre 11 to which new portions of the grating are written.Alternatively, the analyser 16 can be connected to the other end of thefibre, or two analysers can be simultaneously connected at both ends ofthe fibre. The analyser or analysers measure the reflection spectrum ora portion thereof or they measure the transmittance spectrum or aportion thereof or they measure the group delay spectrum or a portionthereof. The analyser 16 is connected to a computer system 17 whichperforms an analysis of the characteristics. The computer system 17 isadapted to select the portion or portions of the reflectance,transmittance or group delay spectra which best monitors the state ofthe grating structure.

The computer system 17 is adapted to control various devices so as tocontrol the phase and intensity of the interference pattern 10. Thesedevices can include the intensity modulator 3, the phase modulator 12and translatable stages as indicated schematically 19, 20 upon which themirrors 8, 9 can be mounted. The computer system 17 can thereby controlthe phase and intensity of the interference pattern in the region 10 atany particular stage. The computer system 17 can utilise varioustechniques in changing the intensity and power of the interferencepattern in the region 10. In a first simple approach, the reflectionspectrum, transmission spectrum group delay spectrum or portion thereofcan be measured and compared with a predicted spectrum. The comparisoncan then be utilised to determine any change in the intensity and phaserequirements which would provide improved results. Of course, thetechnique of maintaining a substantially stable interference pattern inthe reference frame of the fibre 11 is through the saw tooth modulationof the phase delay element 12 as disclosed in Stepanov is preferablyalso utilised.

Alternative techniques can also be employed. For example, the gratingpattern itself can be locally modulated by a small degree in intensityor phase at the point at which the grating is being written. The localwavelength and strength can then be sensed via examination of themodulation reflection spectrum denoted R(z,L_o) and, if required, themodulation group delay spectrum P(z,L_o) where L_o is the wavelength ofthe probe beam utilised by spectrum analyser 16. Preferably, to maintainthe maximum effect, the spectra can be measured from the end of thefibre which is being written such that light does not have to passthrough the previously written grating to reach a region currently beingprocessed by UV radiation.

The response of the spectra will most likely be largest when the localBragg wavelength L is equal to L_o. Hence, dithering of L_o will providea signal which gives a maximum response.

More generally, the predicted response can be readily estimated from theknowledge of the previously written L(z′), K(z′) for z′<z, so that apredictor-corrector algorithm can be run in real time to ensure thatL(z) is exact and the illumination is terminated when K(z) reaches thedesired value.

Given this approach, it is necessary to determine how to apply amodulation of the grating at the point of writing. There are manyapproaches—use of strain, temperature etc which can be locally inducedby a number of means. However, many of these will perturb theinterferometer (vibration, thermal gradients etc,) and this is not wise.The interferometer arrangement of mirrors 8, 9 can be modulated (eg theangle) but this may cause a loss in contrast of the written grating.

The preferred method is to modulate the intensity of the writing UV beamby a small amount using intensity modulator 3. It is thought that thereis both a permanent grating being written as well as a transient grating(at the same pitch) which decays on the time scale of microseconds asthe UV induced defects recombine. Provided that the modulation frequencyis les than the relaxation rate, and larger than the rate of traverse ofthe UV beam over the writing point, then the modulation spectra canprovide an accurate measure of L(z) and K(z) through real timecomputation. There will be a small decrease in the transient response asthe grating saturates, but this should be calibrated for. Intensitymodulation can be added to the UV beam by adding a high frequencymodulation to the AO modulator 3 (or whatever is used).

Alternatively, the spectrum analyser 16 could be simplified such thatthe average spectrum was measured along with the modulation component(by phase sensitive detection). These signals would be measured as afunction of the probe wavelength. Depending on the response function,the probe wavelength may have to be dithered (eg to find the maximumresponse). There are likely to be points where the sensitivity tomodulation may be small due to the influence of the previously writtengrating. An approach is to set the probe wavelength to the desired Braggwavelength and change the interferometer to maximise the modulationsignal. The DC and AC reflectance can then be used to determine the UVdose The group delay modulation can be measured using an interferometricmeasurement.

The alternative approach would be to measure using an appropriate doublemodulation approach whereby the wavelength (or phase) of the source z ismodulated at a frequency θ₁ around a set point (determined from thetheoretical spectra) and the modulation response is measured to enhancesharp features, and the grating at the point of writing is modulated atfrequency θ₁ by any means which changes the local index at the point ofwriting, and the reflected signal is phase sensitive detected at the sumor difference frequencies (θ₁±θ₂). This generic approach to reflectionspectroscopy was developed by Manuel Cardona.

It may be the spectral difference of Fabry-Perot resonances is required,and this could be implemented using a matched Fabry-Perot. The doublemodulation technique described above gives a spectrum which is sensitiveto the region in which the spectrum is being written, and its effect onsharp features (eg Fabry-Perot resonances).

The principles aformentioned can be readily extended to otherinterferometric writing arrangements. For example, in FIG. 2, there isillustrated and arrangement 30 which is derived from the aforementionedOuellette et. al. reference as disclosed by the aforementioned Stepanovet. al. reference. In this arrangement, the elements having the samefunction as in FIG. 1 have their numbers retained. In the arrangement30, the difference is that a Sagnac type interferometric arrangement isutilized wherein a phase mask 31 splits a coherent input beam into twoseparate beams one of which is phase moduilator 12 with respect to theother as is taught by Stepanov et. al. However, the principles of FIG. 1are extended to this arrangement wherein the fibre 14 is interconnectedto an analyser 16 which is in turn interconnected to a computer system17 which controls the translation 19, 20 of the mirrors 8, 9 in additionto the intensity via intensity modulator 3 and the phase via phasemodulator 12 so as to provide for an improved grating. The real timecontrol principles as aforementioned can be utilized in thisarrangement.

It would be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

We claim:
 1. A method of forming a complete grating structure in aphotosensitive waveguide comprising the steps of: (a) writing an initialportion of said structure in said waveguide; (b) measuring lightpropagating properties of said initial portion in said waveguide todetermine a series of parameters of said initial portion; (c) utilizingsaid parameters as a feedback to alter the character of a subsequentlywritten portion of said grating structure to provide foran improved formof grating structure; (d) iterating said steps (a) to (c) so as to formsaid complete grating structure.
 2. A method as claimed in claim 1wherein said writing is performed utilizing a coherence pattern formedfrom the interference of two coherent beams on said waveguide.
 3. Amethod as claimed in claim 1 wherein said characteristics include theintensity or phase of said subsequently written portion.
 4. A method asclaimed in one of claims 1-3 wherein said testing includes determiningthe spectral reflectance response, or part thereof, of said initialportion or subsequently written portions, prior to completion of saidgrating structure.
 5. A method as claimed in one of claims 1-3 whereinsaid testing includes determining the spectral transmittance response,or part thereof, of said initial portion or subsequently writtenportions, prior to completion of said grating structure.
 6. A method asclaimed in one of claims 1-3 wherein said testing includes determiningthe phase delay spectral response, or part thereof, of said initialportion or subsequently written portions, prior to completion of saidgrating structure.
 7. A method as claimed in one of claims 1-3 whereinsaid step (a) further comprises the step of locally perturbing theintensity, phase or frequency of said grating structure and said step(b) comprises measuring said local perturbation.
 8. A method as claimedin claim 7 wherein said local perturbation comprises a periodicmodulation.
 9. A method as claimed in claim 8 wherein said testing stepfurther comprises measuring sum and difference frequencies for saidmodulation.
 10. A method as claimed in one of claims 1-3 wherein saidtesting comprises averaging portions of the reflectance, transmittanceor group delay spectra of said initial portion and of subsequentlywritten portions, prior to completion of said grating structure.